1. Field of the Invention
The present invention relates to the connections between an uncooled single-wall pipe and a cooled double-wall pipe to be used for realizing double-pipe heat exchangers termed also Linear Quench Exchangers (LQE). More generally, the present invention also relates to double-pipe exchangers including said connections.
2. State of the Prior Art
In the prior art, the problems had at connections between cooled pipes and uncooled pipes in double-pipe LQE heat exchangers are known. Indeed, the cooled pipe is typically realized with two coaxial pipes with the innermost of these defining in it a duct run through by the hot fluid (for example gas coming out of a furnace), while the outermost delimits the hollow space in which runs the cooling fluid (for example water). Said cooling fluid is let into the hollow space through a connection in the side wall of the external jacket near the inlet end of the cooled double-wall pipe. Then, typically, the cooling fluid is taken from the hollow space near the outlet end of the double-wall pipe. This “double-wall” structure upstream must be connected with an uncooled “single-wall” pipe carrying in it the hot fluid to be cooled and which therefore is at a relatively high temperature.
To achieve connection between the pipes, a connection member with a first single-wall tubular end on which is welded the uncooled pipe and a double-wall opposite end with two concentric walls on which are welded respectively an internal pipe and an external jacket of the cooled double-wall pipe is generally used. The longitudinal cross section of the connection member can be assimilated with that of a fork.
In this type of connection between a hot single-wall inlet pipe and a cooled double-wall pipe the walls of the pipes and the forked connection in the contiguous points have different temperatures that produce swelling harmful for the strength of the coupling. In addition, the welds between the cooled pipe and the forked connection are in a not well-cooled zone since the connection for cooling fluid inlet to the hollow space is located further along in the cooled pipe.
In the prior art it was sought to remedy this situation by placing in the internal zone of the fork a refractory material which would reduce heat transmission toward the weld zones.
It was also proposed to connect the forked member and the cooled pipe only at the external jacket of the cooled pipe so as to allow independent thermal swelling for the inner parts of the pipe and the connection which are in contact with the hot fluid. This of course requires that the cooling hollow-space in the cooled pipe be closed at the end of the pipe before it reaches the connection member. Otherwise, the hydraulic seal between the cooled-pipe interior (run through by hot fluid) and the hollow space of the cooling fluid would be lacking. In addition, another hollow space more or less open to the hot fluid and containing the refractory would be created, would disturb the flow and might set off coke formation.
But in addition to the complexity and cost of realization, such a solution suffers from periodic breakage of the refractory material which must therefore be replaced with not negligible work and cost. In addition, the fork branches receiving the insulation must be relatively long to receive insulation material in adequate quantities and move the connection between the fork connection and the cooled pipe far enough from the hot zone. This involves further increase in the costs of the connection since due to its nature the forked member must be made of better and therefore more costly material compared to the rest of the structure.
Heat exchangers of the LQE type, in addition to problems related to the connection between the uncooled inlet pipe and the cooled double-wall pipe, also display some problems at the connection between the double-wall pipe and the cooled single-wall fluid outlet pipe. Indeed, near these connections a side passage designed to supply a way out for the cooling fluid that has run through the hollow space of the exchanger is created on the outer wall of the double pipe. The cooling-fluid side passage outlet in prior art heat exchangers is arranged upstream of the terminal part of the hollow space along the double-wall pipe. This fact can lead to the formation of steam bubbles downstream of the outlet passage or, in any case, to the formation of cooling fluid stagnation zones on the bottom of the hollow space, thus nullifying the good operation of the heat exchanger.
The general purpose of the present invention is to remedy the above mentioned shortcomings by making available a stout and relatively economical connection between an uncooled single-wall pipe and a cooled double-wall pipe at the inlet of the LQE heat exchangers.
Another purpose of the present invention is to make available connections between a cooled double-wall pipe and an uncooled pipe which would avoid the formation of cooling fluid stagnation zones or steam bubbles in the hollow space of the cooled pipe.